This invention relates to rugged, ultrasonic transducers and, more particularly, to ultrasonic transducers of the capacitance or SELL-type. Capacitance OR SELL-type electrostatic transducers are well known in the art. They typically comprise a transducer having a thin plastic insulating film metallized on one surface to form an electrode. The metallized film is stretched over a relatively massive metallic counter electrode, sometimes termed the backplate, with the non-metallized, non-conductive surface of the film in contact with the surface of the counter electrode or backplate. The metallized surface of the film separated by the insulating film from the backplate to form an ultrasonic transducer. Typically, such backplates are grooved. See Muggi et al. U. S. Pat. No. 4,081,826, Reynard U.S. Pat. No. 4,311,881, and Grebeler U.S. Pat. No. 4,887,248. The grooving or structural formations in the counter electrode or backplate is preferably eliminated in the present invention and backplate or counter electrode is made of a gas permeable sintered metal conductive disk member.
In a preferred embodiment, the edge of the gas permeable sintered metal conductive back electrode member is machined to provide an annular groove or shoulder in which is seated a smooth plastic ring for clamping the film while it is stretched across the surface of the sintered metal conductive disk member serving as a back electrode or backplate member.
Accordingly, the object of the invention is to provide a rugged, low cost ultrasonically efficient transducer. An ultrasonic transducer according to the invention comprises a hollow cylindrical housing, which may be conductive or nonconductive, and having an interior surface and means forming an annular shoulder. If the hollow cylindrical housing is nonconductive, then a conductive electrode is placed or formed on the annular shoulder and coupled through an electrical connector to serve as the ground electrode for the transducer. A high temperature plastic insulating film, such as Kapton, has first and second sides and a conductive metal film on the first side is engagable with the annular shoulder or the conductive means on the annular shoulder. A gas permeable sintered metal conductive disk member serving as a backplate or counter electrode has third and fourth sides, with the third side contacting the second side of the insulating film. Springs, in the form of a stack of Belville washers apply pressure on the fourth side of the gas permeable sintered metal disk to thereby clamp the high temperature plastic insulating film in stretched relation over the one surface of the sintered metal disk. A signalling or hot conductor is connected to the sintered metal conductive disk. The transducer is broadband with coarser screen mesh sizes of metal particles in the sintered metal having a lower frequency of operation; super fine mesh sizes at the highest frequency.